Deschutes & Ochoco National Forests
Crooked River National Grassland

Geology Summary A Brief Summary of the Geology of Central Oregon Larry Chitwood, Forest Geologist Deschutes National Forest July 1997   A glance at a geologic map of central Oregon shows that volcanoes dominate the landscape and have been erupting here for at least the past 45 million years. The youngest known eruption in central Oregon produced the Big Obsidian Flow in Newberry Crater 1300 years ago. There are no signs that volcanism will let up in the future. Warm Seas of Oregon Until about 70 million years ago, most of what is now Oregon was covered by warm seas that supported a rich variety of sea life including brachiopods, corals, sponges, and ammonites. On low-lying land, ferns, cycads, ginkgoes, and conifers grew in a warm, temperate climate. Fossils of these and many other plants and animals have been found in the older rocks of Oregon.

GEOLOGIC SITES GEOLOGIC INFO Geologic Summary Geomorphic Provinces Geothermal Energy Holocene Eruptions Local Earthquakes Oregon Volcanoes Soils IMAGE GALLERY

Volcanoes Begin to Erupt

From about 70 million years ago to the present, land emerged from the tropical warm seas and volcanism dominated the geologic history of Oregon. The Cascade Range, a great north-south chain of volcanoes, has been growing in episodes for the past 40 million years. Fifteen million years ago they had grown high enough to affect the climate of central and eastern Oregon. The Cascades began to block moisture carried eastward by winds from the Pacific Ocean. Today, the climate east of the Cascades is relatively dry (mostly 10 to 25 inches of precipitation annually), and west of the Cascades it is wet (40 to 100 inches).

The Clarno and John Day Formations

Volcanoes and uplift of the land drove the seas out of Oregon. The semi-tropical climate was warm and moist as shown by fossils and clay soils from the famous Clarno and John Day Formations of central and eastern Oregon. Palms, avocados, pecans, and walnuts grew, and four-toed horses, rhinos, tapirs, and crocodiles inhabited swamps and land. Later, saber-toothed cats, tiny camels, and giant pigs roamed parts of this area.

The John Day Formation contains colorful deposits of ash that erupted in the Cascades and carried eastward by winds. Most of the glass shards that make up the ash layers have turned into clays, zeolites, and opal.

The Clarno Formation formed from about 44 to 40 million years ago; the John Day Formation from 36 to 18 million years.

Floods of Lava

Beginning 17 million years ago, about when rainfall began to decrease, volcanic activity sent enormous floods of lava, called the Columbia River Basalts, over the landscape east of the Cascades. For the next 5 million years, flood after flood of lava buried everything except the highest hills and mountains. All plants and animals in the path of these lavas were killed and buried. Enough time passed between some flows for plants and animals to re-establish themselves. Volcanism before and after these enormous outpourings was common but much smaller in scale.

Uplift

During the past few million years, the Blue Mountains rose up to their present elevation. They were uplifted several thousand feet along a northeast-southwest axis, from the Wallowa Mountains to Smith Rock State Park. This great uplift is called the Blue Mountain anticline. In the middle of the anticline, where uplift and erosion is greatest, are found the oldest rocks in Oregon (380 million year old chert).

New Wildlife and Plants

After the Cascades grew and blocked moisture, the animals of an earlier time were replaced by antelope, deer, bear, modern horses, and mastodons. Plants were mostly replaced by those we see today.

Recent Growth of the Cascades

The Cascades have grown in several episodes of volcanism from 40 million years to the present. The last two episodes created the view of the Cascades seen from eastern Oregon (from about 8 to 5 million years, and from about 2 million years to the present). Green Ridge (north of the town of Sisters) and the lowlands to the east were the product of the earlier episode. In the lowlands, several hundred feet of sand, gravel, ash, pumice, and lava accumulated from eruptions and erosion in the Green Ridge area. These deposits make up the Deschutes Formation displayed dramatically in the canyon walls of Cove Palisades State Park.

The High Cascades

The most recent episode of volcanism produced the High Cascades and the high volcanic peaks perched on the crest such as the Three Sisters, Broken Top, and Mt. Jefferson. The High Cascades consist mostly of large numbers of overlapping shield volcanoes. An excellent example of a shield volcano is Belknap Crater volcano along the McKenzie Pass Highway (State Highway 242). At McKenzie Pass, great fields of rough, black lava, 1500 years years old, form a flattened cone 4 miles in diameter with Belknap Crater at its center. Some lava has flowed many miles beyond the cone. Elsewhere in the Cascades the chemistry of the lavas and short length of lava flows created much steeper and taller cones such as Black Butte and Odell Butte. South Sister, Broken Top, Mt. Jefferson, and several other peaks have a more complex volcanic history.

Upper Deschutes River

The Upper Deschutes River has a long history and has been shaped by a great diversity of geologic events and processes. The primary processes involve volcanism, glaciation, and tectonism. Newberry Volcano and the Cascade Range have flooded the area repeatedly with lava and other volcanic deposits, and great ice age glaciers have discharged large volumes of water, sand, and gravel into the river. The Deschutes River has adapted to the rapidly changing conditions of this volcanic and glacial landscape.

Newberry Volcano

South of Bend is Newberry Volcano, one of the largest "young" volcanoes in the lower 48 states. It covers 500 square miles and rises to 7985 feet elevation at the top of Paulina Peak (a spectacular vista point reached by car on a dirt road). From the top of Pilot Butte in Bend, Newberry Volcano fills the entire view to the south. About 3/4 of the great bulk of Newberry is made up of lava flows and cinder cones; the remaining 1/4 is ash-flow and air-fall deposits. The last two are products of violent eruptions. Some types of molten rock (dacite and rhyolite) are stiff and often highly gas-charged. If dacite or rhyolite containing little gas erupts, the result is a dome or short thick lava flow too stiff to travel any distance. An example is the dome that formed in the crater of Mt. St. Helens during the 1980s. If the stiff molten rock is gas-charged, the result is a towering eruption of ash and pumice. Winds may blow the ash and pumice across large areas downwind of the eruption producing air-fall deposits. Sometimes the ash and pumice flow across the land in devastating clouds of hot gases (up to 1500 degree F) that often travel at hurricane speeds. These avalanches of ash and gases leave behind sheets of ash and pumice which, if hot enough, weld together to form welded tuff.

Newberry Crater and Obsidian

At the broad top of Newberry Volcano is Newberry Crater, a 5-mile-diameter depression, a caldera, which holds two scenic lakes and a variety of young volcanic features. Like Crater Lake to the south, the top of the mountain collapsed after repeated large and violent eruptions during the past half-million years. The most recent events have been smaller eruptions of pumice (a volcanic foam that floats), and stiff, thick flows of black glass (obsidian) between 7300 and 1300 years ago. During the most recent eruption, west winds spread pumice and ash over the land for tens of miles to the east. The floor of Newberry Volcano is rising; it has risen more than 20 feet in 1500 years.

Lava Butte

About 7000 years ago, a dozen or so lava flows and cinder cones erupted from fissures on the flanks of Newberry Volcano. An excellent example is Lava Butte, a 500-foot-high cinder cone south of Bend along Highway 97. A road spirals to the top providing a grand vista of volcanic country. Here, gas-charged molten rock sprayed volcanic foam (cinders) into the air. These fell back into a pile to form Lava Butte. As the eruption proceeded, the amount of gas (mostly water vapor) contained in the molten rock decreased and lava poured out the south side of Lava Butte and flowed 6 miles downhill. The lava spilled into the nearby Deschutes River forming lava dams in some places and shoving the river westward out of its channel in others.

Lava Flows and Lava Tubes

Lavas from Newberry Volcano and a few other sources cover large areas east and north of Bend. Some poured into canyons and flowed down them for tens of miles. Most of these lavas are a few hundred thousand years old and retain original surface features such as tumuli (lava blisters), pressure ridges, and pressure plateaus. Many contain caves called lava tubes. Lava once flowed through these tubes to feed the spreading front of the lava flows. A good example is Lava River Cave along Highway 97 south of Bend.

Volcanic Ash

Soils over much of the land and in the caves of central Oregon are derived mostly from volcanic ash. The last major contribution came 7700 years ago when Mt. Mazama (at Crater Lake National Park) exploded sending 50 cubic miles of pumice and ash into the air. Winds carried and distributed ash over Oregon, Washington, Idaho, Montana, British Columbia, and Alberta. Today, plants, animals, and people depend in many ways on this important ash. For archaeologists, the ash can be extremely useful. Artifacts found below the ash are strong evidence that people occupied a site before 7700 years ago.

Glaciers

During worldwide Ice Ages in the past 800,000 years, glaciers advanced and retreated several times in the Cascade Range. With each advance, glaciers cut deeply into the Cascades leaving impressive canyons and hollowed-out volcanoes such as Broken Top. The most recent major advance reached a maximum about 22,000 years ago. Ice thickened to perhaps 2000 feet along the Cascade crest and glaciers reached to within 7 miles of what is now Bend.

Pluvial Lakes

To the southeast, lakes filled basins between fault-block mountains because temperatures during the Ice Ages were lower by 5 to 15 degrees F and precipitation was slightly higher. These lakes, called pluvial (rain) lakes, often supported fish, shellfish, and other freshwater aquatic life. Mammoths, horses, camels, and other land animals flourished along their shores. At prehistoric Fort Rock Lake, 60 miles southeast of Bend, native Americans made temporary homes in shallow caves along its shore 15,000 years ago. A few salty remnants of these lakes remain, such as Lake Abert and Alkali Lake, both north of Lakeview, Oregon.

A Future of Volcanism

The geologic future of central Oregon seems clear: more volcanoes. They seem to erupt about every 500 years. The last eruption was 1300 years ago. The most likely areas for future eruptions are Newberry Volcano (especially Newberry Crater) and the South Sister area.

SELECTED REFERENCES

Most of these references are rather technical although Geology of Oregon and Fire Mountains of the West were written with the layman in mind.

Bacon, C.R. 1987. Mt. Mazama and Crater Lake, Oregon: Geological Society of American Centennial Field Guide - Cordilleran Section, Geological Society of America, p. 301-306.

Harris, S.L. 1988. Fire Mountains of the West: Mountain Press Publishing Co.

Johnston, D.A. and J. Donnelly-Nolan (eds.). 1981, Guides to Some Volcanic Terranes in Washington, Idaho, Oregon, and Northern California: U.S. Geological Survey Circular 838.

Lidstrom, J.W. 1972. A New Model for the Formation of Crater Lake Caldera, Oregon: PhD. thesis, Oregon State University.

MacLeod, N.S., D.R. Sherrod, L.A. Chitwood, and R.A. Jensen. 1995. Geologic Map of Newberry Volcano, Deschutes, Klamath, and Lake Counties, Oregon: U.S. Geological Survey Miscellaneous Geological Investigations Map I-2455, scales 1:62,500 and 1:24,000.

Orr, E.L., W.N. Orr, E.M. Baldwin. 1992. Geology of Oregon (4th ed.): Kendall/Hunt Publishing Co., Dubuque, Iowa.

Peterson, N.V., E.A. Groh, E.M. Taylor, and D.E. Stensland. 1976. Geology and Mineral Resources of Deschutes County, Oregon: Oregon Department of Geology and Mineral Industries Bulletin 89.

Walker, George W. and Norman S. MacLeod, 1991, Geologic Map of Oregon: U.S. Geological Survey, scale 1:500,000.

12/97

USDA Forest Service - Deschutes & Ochoco National Forests
Last Modified: Wednesday, 26 November 2003 at 12:01:49 EST